Researchers at the Massachusetts Institute of Technology (MIT) have developed a glass coating that prevents fogging without distorting its optical properties.
The team, led by Michael Rubner, TDK Professor of Polymer Materials Science and Engineering, developed a method for testing different coatings and materials to see how effective they were at preventing glass from fogging or frosting up.
Their new approach has been published in a paper in the journal ACS Nano, written by Rubner, who is also director of MIT’s Center for Materials Science and Engineering; Robert Cohen, the Raymond A. and Helen E. St. Laurent Professor of Chemical Engineering; doctoral student Hyomin Lee; and recent MIT graduate Maria Alcaraz.
“When people want to tackle the fogging process, caused when microscopic water droplets condense on a cold surface and scatter light, the common way of doing it is to build a surface that’s so hydrophilic - water-loving - that the water spreads out into a sheet,” Rubner said.
“So even though the water’s there, it doesn’t scatter the light.”
However, this approach can be problematic as in applications where it’s important to get an undistorted view, such as cameras or other optical systems, the view can be quite distorted if the thickness of the layer of water varies considerably.
In addition, if the surface is cold, the water on the surface can begin to freeze, forming a frost layer that scatters light, Rubner says.
“If you’re going to have a sheet of water, how do you prevent it from freezing?” Rubner added.
For that purpose, a coating is needed that can absorb a lot of water in a form that cannot freeze.
In fact in many applications it would be useful to have both hydrophobic and hydrophilic traits in the same material, which is what the team did.
They coined the term "Zwitter wettability" to describe this hybrid property.
Rubner said that Zwitter is a German word for hybrid, used in a number of chemistry terms to describe something that carries two opposite properties at once.
In this case, it describes a surface that has the ability to behave as both hydrophobic (to water droplets) and hydrophilic (to gas-phase water molecules).
The surface is made by a process called layer-by-layer deposition.
In this case, alternating layers of two different polymers — poly(vinyl alcohol) and poly(acrylic acid) — are deposited on a glass surface.
“The magic of what we do is nanoscale processing,” Rubner explained, by producing the layers so as to control their properties almost down to the level of individual molecules.
This production process appears relatively easy and inexpensive to carry out on large scales.
“These are common polymers,” Rubner said.
“They’re well-known and cheap, but brought together in a unique way.”
To test the effectiveness of this material, and that of many other alternatives, the team devised a set of extreme tests.
For example, they kept samples of the material at minus 20 degrees Celsius for an hour, then exposed them to a very humid environment.
While untreated glass, or glass treated with conventional hydrophilic or hydrophobic coatings, quickly develops a layer of frost following such treatment, glass with the new treatment remains clear.
However, it still appears to be hydrophobic in the presence of large water droplets.
Lee said the researchers photographed the glass slides under carefully controlled conditions to measure its performance.
“We developed a protocol that allows us to detect how good one coating is in comparison with another,” he said.
Previous testing typically measured the light transmitted through the glass after exposure to humidity, but failed to measure the level of image distortion caused by water condensation.
“We came up with a way to measure them not just for transmission, but also distortion,” Lee said.
While the new coating outperforms others, it does have one drawback: It’s vanishingly thin, so could be vulnerable to aggressive cleaning or mechanical challenges.
For this reason, it may not be useful for applications where it is exposed to harsh environments or to excessive wiping.
Another limitation is that the new coating only prevents small amounts of frost buildup; it wouldn’t work where there’s a continuous source of cold water, such as for deicing an airplane wing, Rubner said.
However that leaves many possible uses: the inside of automobile windshields, safe from both weather and windshield wipers; the inside of supermarket refrigerator cases; and optical systems used in research or in photography.
The coating could also be useful on the inner surfaces of double-pane windows, which can become fogged if even a small leak allows outside air into the sealed space.
Joseph Schlenoff, a professor of polymer science at Florida State University, said: “Everyone knows how inconvenient, or even dangerous, it is to have a cold window or lens fog up when water condenses on it.
"The MIT group has devised a practical and effective method of combatting the fogging problem using a new ultrathin polymer film.
"Both the materials themselves and the techniques used to explore their properties are highly innovative. These MIT engineers are literally helping us to see technology more clearly.”
The researchers' work was supported by Samsung and by the National Science Foundation.
